The present invention relates to the manufacture of semiconductor integrated circuits (ICs) and more particularly to an improved method of depositing a boro-silicate-glass (BSG) layer onto a semiconductor structure. The method finds a valuable application in the fabrication of the buried plate in deep trench storage capacitors.
In the manufacture of semiconductor integrated circuits and particularly in dynamic random access memory (DRAM) chips, deep trenches are extensively used. As known for those skilled in the art, in DRAM chips, an array transfer transistor, typically an insulated gate field effect transistor (IGFET) and a storage capacitor are associated to form the elementary memory cell. To date, due to scaling reduction effects, the storage capacitor is formed in a deep trench etched in the silicon substrate and includes a buried plate region to improve the capacitor dielectric robustness. In the course of the buried plate fabrication process, a layer of BSG is used as a hard mask during the trench formation. The step of removing said BSG layer is an essential part of the whole buried plate fabrication process.
A conventional buried plate fabrication process is described herein below in conjunction with FIG. 1 and FIGS. 2A-2G. All the processing steps are conducted in the so-called deep trench (DT) module.
FIG. 1 schematically illustrates the starting structure referenced 10 which basically consists of a P-type doped silicon substrate 11 coated with a 10 nm thick silicon oxide (SiO2) layer 12 and a 185 nm thick silicon nitride (Si3N4) pad layer 13. These layers are typically deposited by a low pressure chemical vapor deposition technique (LPCVD).
Now turning to FIG. 2A, a 700 nm thick boro-silicate-glass (BSG) layer 14 is blanket deposited onto the P-type silicon substrate 11 by LPCVD as standard. For instance, the BSG layer 14 is deposited in a LAM 9800 tool, manufactured by LAM Research, Fremont, Calif., USA, using tri-ethyl-boron (TEB) and tetra-ethyl-boro-silicate (TEOS) gases according to the following operating conditions:
Next, a photoresist layer (not shown) is deposited onto structure 10, which is baked, exposed and developed as standard according to the desired trench pattern. After resist patterning, the BSG, Si3N4 and SiO2 materials of layers 14, 13 and 12 are sequentially etched, for instance, in the MxP chamber of an AME 5000 plasma etcher, a tool manufactured by Applied Materials, Santa Clara, Calif., USA with the following operating conditions.
(sccm denotes standard cubic centimeter per minute).
The resist mask is then stripped by ashing in a FUSION ACU, a tool manufactured by FUSION, Rockville, Md., USA using an 02/N2 mixture as standard. Now the deep trench is etched in the silicon substrate 11. To that end, the BSG layer 14 is used as a hard mask during trench formation. For instance, the trench can be etched in a TEL 88 DRM tool, manufactured by TOKYO ELECTRON Limited, Yamanachi, Japan.
The following operating conditions are adequate.
The BSG material has to be resistant to the silicon dry etch chemistry, in this case HBr and NF3 which are known to be very aggressive. For instance, with the operating conditions described above, less than 500 nm of BSG material are consumed in the etching of a trench having about 7-8 xcexcm depth. At this stage of the buried plate fabrication, the structure 10 appears as shown in FIG. 2A with a deep trench 15 formed therein.
The BSG layer 14 must be stripped for reasons given later on. Typically, it is stripped with a conventional HF vapor wet process in an EXCALIBUR tool sold by FSI, Chaska, Minn., USA with N2 and HF flows of 60 l/min and 1.5 l/min respectively at 65xc2x0 C. The etch rate is about 200 nm/min. The resulting structure is shown in FIG. 2B. This wet etch process is essentially isotropic, so that an undesired undercut referenced 16 in FIG. 2B is formed on the SiO2 layer 12 side exposed in the trench 15.
Because, this undercut 16 would be detrimental to the buried plate fabrication process, the Si3N4 layer 13 needs to be partially etched (14 nm) to avoid an excessive overhang above the SiO2 layer 12 and subsequent polysilicon fill problems. This step which is typically performed by wet chemistry is usually referred to as the pullback step in the technical literature. For instance, it can be performed in a MAGNUM SAT wet bench, a tol manufactured by SEMITOOL, Kalispell, Mont., USA using a HF/glycerol bath (1:25 ratio) wherein the HF is diluted in water (49/51 in volume %). FIG. 2C shows structure 10 at this stage of the buried plate fabrication. As apparent in FIG. 2C, thanks to the pullback step, the undercut effect has been clearly reduced.
Now, referring to FIG. 2D, a 400 nm thick arsenic silicon glass (ASG) layer 17 is first conformally deposited onto the structure 10 to entirely coat the trench 15 side wall. In turn, a 2 xcexcm thick resist layer 18 is blanket deposited onto the structure 10 to fill the trench 15.
The resist layer 18 is recessed down to a deepness of 1.3 xcexcm in trench 15 using a O2/CF4 chemistry as standard in an AME 5000 plasma etcher mentioned above, so that the ASG layer 17 is exposed in the upper part of the trench.
Working conditions are:
This step is monitored by an etch end point system. First, in the resist planarization step, exposure of the Si3N4 etch stop layer 13 is detected by the CN radiation (388 nm). Next, the signal generated by an interferometer such as the MULTISEM 550, an equipment manufactured by SOFIE Inst., Arpajon, France, is monitored to measure the resist amount etched in the trench. The etch process is stopped when the desired depth of 1.3 xcexcm is attained. This step which is thus only monitored by time is very critical to the whole BP fabrication process. A non-uniform BSG layer 14 thickness will be transferred to the resist level 17A which is determining to the BP level definition.
The ASG material of layer 17 which thus becomes exposed is removed in a BHF bath (NH4F/HF/water in the 5:1:48 ratio in volume). Finally, the resist material remaining in trench 15 bottom is stripped as standard. The resulting structure is shown in FIG. 2E.
A TEOS SiO2 layer 19 having a thickness of 50 nm is conformally deposited by LPCVD to coat structure 10. Then, a thermal treatment is performed in a furnace at 950xc2x0 C. to drive arsenic atoms out of the remaining ASG layer 17 into the substrate 11 to form the buried plate (BP) region 20 as illustrated in FIG. 2F.
Now, the ASG and TEOS materials of layers 17 and 19 are removed from the trench 15 with a conventional wet process using a diluted HF (DHF) solution. As apparent in FIG. 2G, a dual nitride-oxide (NO) dielectric film 21 is conformally deposited onto the structure 10 to entirely coat its surface including the trench 15 sidewall. Finally, doped polysilicon is blanket deposited by LPCVD onto structure 10 to fill the trench 15. The doped polysilicon layer referenced 22 in FIG. 2G is planarized by chemical-mechanical polishing using the Si3N4 pad layer 13 as an etch stop layer.
The BSG layer 14 has to be stripped before ASG layer 17 is deposited because it has a very non uniform thickness after the trench formation process described by reference to FIG. 2A (typically the BSG layer 14 thickness varies from 120 to 400 nm across the wafer). If it is not stripped, this non-uniformity will be transferred during the resist layer 18 recessing (described above by reference to FIG. 2D) so that the formation of the buried plate region 20 is very dependent of the level referenced 17A in FIGS. 2E and 2F of the remaining ASG layer 17 in trench 15. In particular, if level 17A is too low, ASG layer 17 will not play any longer its protective role of the capacitor dielectric. As a result the dimensions of buried plate 20 will also be non-uniform which is detrimental to the whole buried plate fabrication process reliability.
The above buried plate fabrication process which thus requires to remove the BSG material of layer 14 (as described by reference to FIG. 2B) suffers from three main concerns.
1. The BSG layer 14 wet etch strip is done at the initial stage of the trench formation process when the trench is not filled with the polysilicon of layer 22. This wet etch process is essentially isotropic, so that an undesired undercut 16 is formed on the SiO2 layer 12 exposed sides. BSG material of layer 14 is deposited at 780xc2x0 C. and has a low wet etch rate so that this undercut 16 in the thin SiO2 layer 12 is important.
2. A pullback step is required to reduce the overhang in the Si3N4 pad layer 13 to avoid subsequent polysilicon fill problems.
3. Even if we are able to perform this BSG layer 14 wet etch properly, we are drastically decreasing the process window for the other wet steps in the DT module, and in particular, the conventional wet processes using a diluted HF solution that remove the materials of layers 17 and 19 from the trench. Missing these steps will leave some ASG and TEOS residues on the trench 15 sidewall and change the capacitor dielectric composition from NO to ONO increasing thereby the dielectric film 21 thickness. As known to those skilled in the art, an increase in the capacitor dielectric thickness will decrease the overall trench capacitance (retention time) which is detrimental to memory cell device performance.
It is therefore a primary object of the present invention to provide an improved method of depositing a boro-silicate-glass (BSG) layer onto a semiconductor structure that is resistant to silicon dry etch chemistries and can be easily and rapidly stripped in standard BSG etchants.
It is another object of the present invention to provide an improved method of fabricating the buried plate in a deep trench cell capacitor comprising the step of depositing a boro-silicate-glass (BSG) layer onto a semiconductor structure that is resistant to silicon dry etch chemistries.
It is another object of the present invention to provide an improved method of fabricating the buried plate in a deep trench cell capacitor comprising the step of depositing a boro-silicate-glass (BSG) layer onto a semiconductor structure that is easily and rapidly strippable in standard BSG etchants.
It is another object of the present invention to provide an improved method of fabricating the buried plate in a deep trench cell capacitor comprising the step of depositing a boro-silicate-glass (BSG) layer onto a semiconductor structure that is easily and rapidly strippable in a HF/ethylene glycol bath.
It is still another object of the present invention to provide an improved method of fabricating the buried plate in a deep trench cell capacitor comprising the step of depositing a boro-silicate-glass (BSG) layer onto a semiconductor structure which reduces the total number of processing steps thereof.
According to a general aspect of the method of the present invention, the boro-silicate-glass (BSG) layer is deposited onto a semiconductor structure by LPCVD using an O3/TEB/TEOS mixture at a pressure less than 300 mTorr, a temperature less than 500xc2x0 C. and adjusting the TEB flow so that the boron concentration is less than 10% (in weight).
Still according to another aspect of the present invention, there is also described a specific HF/ethylene glycol based etchant that allows a very high etch rate and therefore has a high BSG/SiO2 selectivity.
Still according to another further aspect of the present invention, the above step of depositing a boro-silicate-glass (BSG) layer finds a valuable application in a method of fabricating the buried plate in deep trench cell capacitors comprising the steps of:
a) providing a semiconductor structure consisting of a silicon substrate coated with a bottom SiO2 layer and an overlying Si3N4 pad layer;
b) depositing a layer of BSG material onto the structure by LPCVD using an O3/TEB/TEOS mixture at a pressure less than 300 mTorr, a temperature less than 500xc2x0 C. and adjusting the TEB flow so that the boron concentration is less than 10% (in weight);
c) forming an opening through said BSG, Si3N4 and SiO2 layers to expose the silicon substrate;
d) forming a trench in said silicon substrate using the patterned BSG layer as an in-situ hard mask and an adequate chemistry for silicon dry etching;
e) removing said top BSG layer by a wet process;
f) coating the lower part of the trench with an arsenic-silicon-glass (ASG) layer corresponding to the desired level for buried plate formation; and,
g) depositing a tetra-ethyl-ortho-silicate (TEOS) layer to coat the trench interior sidewall and annealing the structure to outdiffuse the arsenic atoms of the ASG layer into the silicon substrate to form the buried plate.
The novel features believed to be characteristic of this invention are set forth in the appended claims. The invention itself, however, as well as other objects and advantages thereof, may be best understood by reference to the following detailed description of the invention to be read in conjunction with the accompanying drawings.